Gypsum boards are composite materials obtained by lining both surfaces of gypsum core materials with a paper, are produced in an amount of about 4 million tons, and have now been widely used in a variety of fields. On the other hand, the gypsum boards that were once used and discarded (waste gypsum board) are now amounting to about one million tons annularly, and their amount is expected to further increase in the future.
Some of the waste gypsum boards have been recycled, as required, for separating papers therefrom and for recovering the gypsum for use again as the starting material for gypsum boards and for use as solidifying agents. However, the ratio of recovery is still low and most of them have been dumped on reclamation sites. From the standpoints of an increase in the amount of the waste gypsum boards in the future, limited final disposal sites in Japan and burden on the environment, therefore, a new use of the waste gypsum boards and a new recycling method thereof have been urged.
On the other hand, a vinyl chloride resin has a high degree of general applicability and can be blended with suitable amounts of inorganic fillers depending upon the use to attain reinforcement or to increase the amount thereof. By adding inexpensive gypsum recovered from the waste gypsum boards as an inorganic filler to the vinyl chloride resin, therefore, large amounts of the recovered gypsum can be reused.
Here, the gypsum recovered from the waste gypsum boards is a dihydrate gypsum (CaSO4.2H2O) and contains hydrated water. Therefore, a proposal has been made to use the dihydrate gypsum as a foaming agent for producing a foamed formed article of the vinyl chloride resin (see patent document 1). Namely, the dihydrate gypsum develops dehydration when the vinyl chloride resin is being formed enabling the water content to be utilized for the foaming.
There has, further, been proposed an example of blending the vinyl chloride resin with a hemihydrate gypsum as a filler. In this case, too, the obtained formed article is a foamed article (see patent document 2).
There has, further, been proposed a production of a nonfoamed formed article by using a mixture obtained by mixing calcium sulfate as a filler to the vinyl chloride resin (see patent document 3).
However, said calcium sulfate (CaSO4) to be added to the vinyl chloride resin is an anhydrous gypsum which is not the hydrated water-containing gypsum (hereinafter often referred to simply as hydrated gypsum) such as the dihydrate gypsum or the hemihydrate gypsum (CaSO4.0.5H2O). As proposed in the above patent document 1, the hydrated gypsum causes foaming or defective forming due to the formation of water in the step of forming, resulting in defective appearance of the obtained formed articles. In producing the nonfoamed formed article of the vinyl chloride resin, therefore, said calcium sulfate that is added to the vinyl chloride resin is an anhydrous gypsum. When the hydrated gypsum is to be used, therefore, the hydrated gypsum must be converted into the anhydrous gypsum by the dehydration treatment prior to being added to the vinyl chloride resin.
The anhydrous gypsum includes those of the type I, type II and type III. The anhydrous gypsum of the type III having the lowest transition temperature has hygroscopic property, reacts with water in the open air and is easily converted into the hemihydrate gypsum. Therefore, the dihydrate gypsum is once converted into the anhydrous gypsum (non-hygroscopic gypsum) of the type II or the type I and is, thereafter, added to the vinyl chloride resin. Here, however, the dihydrate gypsum can be converted into the anhydrous gypsum of the type II requiring the heating at not lower than 400° C. and can be converted into the anhydrous gypsum of the type I requiring the heating at a further elevated temperature. In either case, therefore, a tremendous energy cost is necessary.
It is not allowed, as a matter of course, to convert the dihydrate gypsum into the non-hygroscopic anhydrous gypsum (type II or type I) in the presence of the vinyl chloride resin. The heating temperature for the conversion is 400° C. at the lowest, which is far beyond the temperature (about 210° C.) at which the vinyl chloride resin starts thermally decomposing.
Though the recovered gypsum is inexpensive as described above, the recovered gypsum which is the dihydrate gypsum could be added as the inorganic filler to the vinyl chloride resin only when the foamed article was to be formed. When the nonfoamed article of vinyl chloride resin was to be formed, however, a tremendous energy cost was necessary. In practice, therefore, the recovered gypsum has not been almost utilized.